Stethoscope Science: Hearing Heart Rates

Introduction
Around Valentine's Day heart-shaped images seem to be everywhere. But our actual hearts are at work all year—every hour, minute and second. The heart works hardest when we physically exert ourselves. How does its beating change? A doctor can figure this out by using a tool called a stethoscope, which is a long, thin plastic tube that has a small disc at one end and earpieces at the other. In this activity you will make a homemade stethoscope and use it to measure peoples' heart rates at rest and after exercising.

Background
You're probably familiar with how a stethoscope is used from visits to your own doctor. To listen to the heart, the doctor puts the stethoscope's flat disc or hollow cup on a patient's body and the earpieces go into the doctor's ears. But how does the stethoscope work? How does it allow the doctor to hear sounds inside of the patient's body?

The disc and the tube of the stethoscope amplify small sounds such as the sound of a patient's lungs, heart and other sounds inside the body, making them sound louder. The amplified sounds travel up the stethoscope's tube to the earpieces that the doctor listens through. Heartbeats can easily be heard using a good stethoscope. Every time a person's heart beats it contracts and acts as a powerful pump, which circulates blood that carries oxygen and nutrients throughout the body.

Materials
• Duct tape or other strong tape
• Scissors
• Plastic funnel
• A cardboard tube from a paper-towel roll
• A volunteer who can safely exercise rigorously for one minute
• Stopwatch or clock that counts seconds

Preparation
• Make sure you have a volunteer who can safely exercise rigorously for one minute.
• Put the narrow end of the funnel into the cardboard tube.
• Using a strip of duct tape or other strong tape, tape the funnel and cardboard tube together. Make sure there are no gaps or spaces where you tape them together.
• Your stethoscope is now ready to use! Practice listening to the heartbeat of a volunteer by putting the funnel on the left side of the volunteer's chest. Make sure the funnel is flat against their chest. Put your ear against the hole at the end of the cardboard tube. Do you hear a heartbeat?
• Before you begin the activity have your volunteer sit quietly in a chair for at least five minutes.
• Tip: If it's noisy or the volunteer is wearing thick clothing, it may be heard to hear the heartbeat, so you may need to adjust conditions accordingly.

Procedure
• After the volunteer has been resting in a chair, listen to the heartbeat and count how many times it beats in 15 seconds.
• Multiply this number by four. This is the resting heart rate of the volunteer in beats per minute (bpm). What is the volunteer's resting heart rate?
• Ask the volunteer to exercise in place for one minute by doing jumping jacks or running in place. Right after the volunteer has stopped exercising, listen to the heartbeat and count how many times it beats in 10 seconds. Why do you think you do this for only 10 seconds instead of 15?
• Multiply this number by six. This is the heart rate right after exercising in bpm. What is the volunteer's heart rate now?
• How did the heart rate change after exercise? Why do you think it changed like it did?
• If a person regularly exercised, how do you think this would change his or her heart rate? How do you think that person's heart rate during rest and exercise would be different? Hint: Think about how regular exercise may change the heart.
• Extra: There are several different ways you can make a homemade stethoscope. For example, instead of a cardboard tube you could use a short piece of garden hose or plastic tubing. Or you could try varying the tube’s length. You could even try different size funnels. What homemade stethoscope design works best? What goes into making a good stethoscope design?
• Extra: In this activity you only tested one person's heart rate but you could try your stethoscope on several different people. How do different peoples' heart rates compare with one another? Do they all change a lot after exercising or do some change only a little bit or not at all? Does the heart rate correlate with a factor such as age, gender or body mass index (BMI)?
• Extra: A person should use at least 50 percent of their maximum heart rate when they are exercising for the activity to qualify as exercise. You can find out more about maximum and target heart rates and then apply this information to create an exercise routine. Based on this information, what physical activities does a given person do that qualifies as exercise?

Observations and results
Could you hear the volunteer's heartbeat using the homemade stethoscope? Did you find that when they exercised their heart rate increased compared with what their heart rate was when they were resting?

When people exercise, their bodies need more oxygen, and consequently their hearts beat faster and their heart rates increase. This is why you most likely found that when your volunteer exercised, their heart rate increased compared with their resting rate. In addition, genetics, gender, age and health all affect people's heart rates. The rates in people who exercise regularly usually will not increase as much during exercise and will return to a resting rate more quickly after exercise is stopped. Regular exercise strengthens the heart so that it does not need to work as hard.

Although you can determine someone's resting heart rate by counting the number of beats in 15 seconds and multiplying by four to get the bpm, to calculate a heart rate immediately after exercise, it is better to count the number of beats for 10 seconds and multiply that value by six (to get the bmp). Because the heart will quickly slow down after exercise ceases, its rate should be measured immediately after a person has stopped exercising (or while they exercise, if possible).